1. Field of the Invention
This invention relates to a method of manufacturing turbine blades, vanes and other turbine components by joining single crystal sections together using a moving zone transient liquid phase bonded (MZTLPB) sandwich construction.
2. Background Information
State-of-the art blades and vanes that are employed in modern, high efficiency power generation combustion turbine engines rely on high quality materials and precise control of the part's internal and external dimensions. Because of the large size of these parts, cost-effective manufacturing is being pursued by several routes. High quality materials have been provided by using single crystal alloys fabricated, for example, as taught by Burke et al. in U.S. Pat. No. 4,637,448. The high performance properties of single crystal alloys are required in the complex cooled structure blades that are to be used for the first and second rows of advanced turbine systems "ATS" and subsequent generation gas turbines. It is difficult, however to incorporate complex cooling channels in the large size land based turbine blade castings without impairing the single crystal quality of the blades. The consequence of these technical difficulties is low yields of castings of high performance single crystals or the need to turn to lower performance SC alloys that may be cast with higher yields.
The concept of fabricating blades from axial planar sections has been identified previously by R. E. Anderson et al. in "Use of RSR Alloys for High Performance Turbine Airfoils", Proceedings of the Second International Conference of on Rapid Solidification Processes, 1980, pp. 416-428. In this concept, highly deformed (rolled) polycrystalline sheets were machined into the shapes of axial section of a blade and were subsequently diffusion brazed together. The excess energy of the surfaces of the thin sheets and the stored energy of the rolled structure provided the driving force for a solid state recrystallization to form a single crystal structure. In this process many thin sections are required to form a blade and each one must be individually machined, especially if internal cooling passages are required, to build up the structure of a blade, adding to costs. Also, the crystallographic texture developed by the solid state recrystallization process may not be as strong or as optimally aligned as the pure &lt;001&gt; single crystals that are developed by casting. In particular, secondary axis orientation cannot be controlled during solid state recrystallization. This process involved solid sate recrystallization of very thin, deformed sheets of nickel base superalloy. Turbine blades having a radial wafer airfoil construction are also shown in U.S. Pat. No. 4,203,706 (Hess). Kingston, in U.S. Pat. No. 5,061,154, also taught joining a plurality of cast single crystal blade segments. Here they were joined by hot isostatic pressing.
Burke et al., in PCT Application No. WO 99/21680, taught making turbine blades made from multiple single crystal, cast, nickel based, superalloy segments joined by a transient liquid phase bonding technique followed by a controlled heat treatment, to produce the desired microstructure in the bond region. Useful known single crystal materials included, for example, a CMSX-4 alloy of Ni with Cr, Co, Al, Ti, Mo, Ta, W, Re and Hf. Boron rich bonding foils, for example, 9 wt % Cr, 8 wt % Co, 4 wt % W, 4 wt % Ta, 1 wt % Hf, 2 wt % Al, 3 wt % B, with the remainder Ni, were used along the entire bonding area for rapid solid state diffusion into the blade segments during bonding, since boron is a meeting point depressant. The bonding process was conducted at a temperature above the melting point of the foil but below the bulk melting point of the blade segments. As the boron rich foil melts, it wets the base material on either side of the bond along the entire bond area, as a single static melt pool, causing some dissolution and a wider liquid zone, lowering the concentration of boron at the same time that solid state diffusion also causes boron loss from the pool environs, allowing resolidification at the centerline of the bond. Potential problems here are a long single molten zone along the entire bonding edge of the blade segment, which is difficult to control. Also, the foils can exhibit limited ductility such that they are difficult to cut into the precise shapes that are necessary to conform to the component surfaces to which they are applied for bonding.
Ryan, in U.S. Pat. No. 4,700,881, improved the transit liquid phase process of bonding turbine engine components together and solved problems of too rapid boron diffusion in fine grained materials by using multiple foils with different amounts of melting point depressants in different layers. The layers are all in place in the same bond gap and all melt at the same time to provide static melt pools all adjacent to each other. In U.S. Pat. No. 4,208,222 (Barlow et al.), the bond material for transient liquid phase bonding of turbine blades is made of a borided portion of a metallic coating on one of the surface to be bonded, providing a boron rich surface for preferential melting. This aids the resolidification process since the molten outer surfaces of the foil better wet the blade section before full melting of the bond foil. Again this provides a static melt pool.
Fitzgerald et al., in U.S. Pat. No. 5,836,075, solved bond foil problem by sputter depositing bonding material on selected opposed mating surfaces of turbine segments to be bonded together, providing well defined and constrained local melting regions, and then bonding the entire bonding edge of the turbine segments by a transient liquid phase bonding technique. This still results in a single, one step melting process, where all portions of the foil are melted at the same time. This can be disadvantageous, because a large molten pool is formed by the large layer of bond material when it is held at a uniform temperature. Because of the large mass of the zone, there is a tendency for the pool to agglomerate in one region, thereby causing excessive local melting and loss of local shape control.
Pfann, in U.S. Pat. No. 2,813,048, taught a zone melting process where a molten region is caused to travel through a body of material while operating at a temperature below the melting point of the body. This involved moving a molten zone, where all the material heated in that zone was in the liquid state, within a solid body toward a region of higher temperature. This resulted however, in cylindrical zones and resultant round solidified solid tubes caused by surface tension of the liquid. This was used to manufacture semiconductors. A liquid-solid interface was always present normal (perpendicular) to the direction of progression of the interface and yielded a high purity, controlled crystal structure. While even quite large diameter materials can be grown in solid sections (that is, greater than 6 inch--15.25 cm--diameter solid tubes in silicon for semiconductors), maintaining internal orifices, such as cooling passages, in a turbine blade would be impossible if a molten zone was passed along a long conventional turbine blade material. What would be needed would be some way to keep separate molten zones from agglomerating. Although a potential answer might be to use internal cores the difficulties of using such a process are expected to be very severe.
There is a need for more cost effective, higher yield processing of single crystal materials. Current casting difficulties on the part of casting vendors causes prices to rise, complex structures are not guaranteed and choices of alloys are restricted to the easier to cast but lower performance SC alloys. One of the problems in the conventional casting industry is that the only time that the product can be inspected for material quality is at the end of the process. Because a considerable fraction of the cost of manufacturing single crystal parts is in the cost of the mold, the cores etc, a great deal of unrecoverable costs are expended before the process "sees metal". Since mold and core problems can also affect parts shape that cannot be discovered until after the part has been fully cast, the conventional casting approach has no way to eliminate unproductive costs by eliminating scrap parts in mid process. A method that would separate the production of single crystal quality and component geometry would be able to reduce costs by inserting intermediate inspection and eliminating unproductive processing of defective materials.
Therefore, what is needed is a new and improved method of bonding single crystal turbine component segments that will allow better control of the bonding process and allow bonding of complex, precise internal surfaces of the turbine component segments.